In the production of carbon artifacts, such as carbon fibers, various chemical and physical processes are used to transform feedstocks and fractions of feedstocks into suitable precursors. Major efforts are being made to obtain a better understanding of the required chemistries.
It has been determined that a deasphaltenated middle fraction of a feedstock can be subsequently heat soaked and vacuum stripped to provide a precursor material suitable for carbon fiber manufacture. Such a teaching is to be found in patent applications assigned to a common assignee, bearing Ser. Nos. 346,623 now U.S. Pat. No. 4,431,512; 346,624 now U.S. Pat. No. 4,427,530 and 346,625 now abandoned, each filed on Feb. 8, 1982.
A polycondensed aromatic feedstock can be transformed into a pitch precursor by a variety of thermal or catalytic processes. For example, Ashland pitch nos. 240,170 or 212 can be produced by a thermal-treatment of catalytic cracking residue. Catalytic cracking residue pitch is produced according to U.S. Pat. No. 4,271,006 by a vacuum heat-soaking process or by heat-soaking a steam cracking tar residue as described in U.S. application Ser. No. 273,200 now U.S. Pat. No. 4,414,095, or by heat-soaking a distillate of a steam cracking tar residue in accordance with U.S. Ser. Nos. 399,751 now abandoned and 346,623 now U.S. Pat. No. 4,431,512 or by heat-soaking distillate from coal processing such as in a coal liquefication process according to U.S. application Ser. Nos. 399,702 now U.S. Pat. No. 4,518,482; 399,472 now U.S. Pat. No. 4,448,670 and 346,625 now abandoned.
An understanding of the characteristics and the chemical structure of the various molecules (or parts) of a pitch precursor is necessary to process and spin the pitch (or fraction) into 8-12 microns "green" fibers. After spinning, the precursor is oxidized (at 220.degree.-300.degree. C.) to transform the "green" fiber infusable, and then, carbonized at 1400.degree.-2000.degree. C. into high tensile strength carbon fibers.
In every instance, to the best of our knowledge and belief, a given feedstock or fraction thereof has been treated or transformed to provide a precursor material for carbon artifact manufacture.
No one, to the best of our knowledge and belief, has ever suggested blending specific fractions or components of one or more pitches to provide a customized precursor.
This invention is based upon a new concept in carbon artifact manufacture, wherein a precursor can be manufactured by blending extracted components of at least one pitch to give an optimized mixture having the proper chemistries and rheology to provide high strength carbon artifacts.
High tensile strength pitch-derived carbon fibers are produced by spinning a carbonaceous material with a specific composition of matter. This composition, generally speaking, comprises two major parts: (1) a low molecular weight, low aromaticity, isotropic, volatile plasticizer part; and (2) a high softening, high aromaticity, thermally-stable, anisotropic part. These two parts must be present in appropriate proportions to produce a molten carbonaceous material with the desired softening, fluidity, rheology, volatility and stability suitable for producing high strength carbon fibers.
More specifically, the invention has further defined and separated individual components within these two major fractions, and has further blended these components in given proportions to provide a customized blend.
Still further, this invention recognizes that individual components can be cross-blended from different feedstocks, e.g. oxidizable components from a coal distillate feedstock pitch can be blended with plasticizers from a cat cracker bottom feedstock pitch.
This invention has realized that not all pitches or fractions of pitches are suitable for carbon fiber production, and that too much or too little of certain components can severely effect the final carbon artifact product.
The invention contemplates custom blending individual pitch components to provide a precursor for carbon fiber production having the following general characteristics:
1. A precursor having highly polycondensed aromatics with minimum alkyl side chains. This characteristic can quantitatively be determined by nuclear magnetic resonance spectroscopy and by measuring the carbon/hydrogen atomic ratio.
2. A precursor having a relatively high molecular weight as measured by gel permeation chromatography.
3. A precursor which is highly anisotropic (high liquid crystal) in structure or is able to transform into a highly anisotropic structure on further heating for a short time in a nitrogen atmosphere.
4. A precursor having satisfactory rheological properties (viscosity or softening characteristics) so it can be spun into 8-12 micro fibers.
5. A precursor which is thermally and chemically stable so it does not decompose or change its chemical structure during spinning at a temperature such as 340.degree.-380.degree. C., and pressure.
6. A precursor having a specific composition of the low boiling volatiles, a low softening plasticizer fraction of one or more components and high molecular weight and high softening fractions which provide the skelton for the carbon fiber.
A typical cat cracker bottom pitch can be separated into eight components or fractions by solvent extraction techniques:
1. Quinoline Insolubles (fraction "O.sub.3 "). Extracted with Quinoline at 75.degree. C.;
2. Pyridine Insolubles--Quinoline Solubles (fraction "O.sub.2 "). Extraction with pyridine at reflux and Quinoline at 75.degree. C.
3. Pyridine Insolubles (fraction "O.sub.2 +O.sub.3 "). Extraction with pyridine at reflux;
4. Toluene Insolubles--Pyridine Solubles (fraction "O.sub.1 "). Extraction with Toluene at reflux and then Pyridine at reflux;
5. Toluene Insolubles (fractions "O.sub.1 +O.sub.2 +O.sub.3 " and "P.sub.1 +P.sub.2 "). Extraction with toluene at reflux;
6. n-Heptane Insolubles--Toluene Solubles (fraction "P.sub.2 "). Extraction by n-Heptane at reflux and then toluene at reflux; and
7. n-Heptane Solubles (fraction "P.sub.1 "). Extraction by n-Heptane at reflux.
The composition analysis is carried out as follows: the material to be tested is introduced into a glass reactor equipped with a mechanical agitator and electrically heated from the outside. The solvent is then added and the mixture agitated vigorously and heated to the desired temperature for the desired time. The insolubles are filtered using fritered glass filters, dried under reduced pressure at around 100.degree. C. and the insolubles yield calculated. Summary of the conditions of the solvent analysis is as follows:
______________________________________ Feed: Time Temperature Test Solvent Ratio (hrs.) (.degree.C.) ______________________________________ n-heptane 1:50 2.0 105 Insolubles Toluene 1:50 1.0 110 Insolubles Pyridine 1:50 1.0 114 Insolubles Quinoline 1:12 4 75 Insolubles ______________________________________
Five of these eight components (O.sub.1 ; O.sub.2 ; O.sub.3 ; P.sub.1 and P.sub.2) can now be individually blended in proportions not commonly obtained in the original pitch.
For example, the "P.sub.2 " component which is usually present in the total cat cracker bottom pitch in a percentage by weight of between 10 and 20%, can now be blended into a customized precursor mixture in a lower percentage such as 5% or a higher percentage such as 30%. The other components can be likewise manipulated.
Thus, a precursor can be fabricated, which has characteristics never before achieved by conventional fractional treatments or by processing.
Even more interesting is the concept of cross-blending pitch components derived from different feedstocks such as: cat cracker bottoms; steam cracker tars; coal distillates; etc.
In addition, synthetic materials may also be added to the customized blend.
Synthetic materials suitable as components for preparing a carbon precursor of our invention can be one or more of the following:
(a) Heat-soaked pitch derived from catalytic cracking residue or its distillate prepared according to the copending application Ser. Nos. 225,060; 399,750 and 346,624.
(b) Heat-soaked pitch derived from steam cracking tar or its distillate according to the process described in the copending application Ser. Nos. 346,623 and 399,751.
(c) Heat-soaked pitch derived from coal tar distillate from coal processing by using the process described in copending application Ser. Nos. 399,472 and 399,702.
The above copending applications are meant to be incorporated herein by way of reference.
Furthermore, in manufacturing processes where precursors are made continuously or in large bulk, the exact characteristics and properties are very difficult to achieve. Small unwanted changes in temperature and pressure may produce a precursor with an unacceptable level of a particular fraction. By custom blending the precursor, this invention can provide the certainty that the correct amount of any component or fraction will be in the final product. Thus, a precursor having specific composition and a final product having exact and specific characteristics can be provided at will.